1. Field of the Invention
The present invention generally relates to the field of optical communications and, more particularly, to apparatus for dispersion compensation.
2. Description of Related Art
Dispersion compensators (DCs) are widely used for compensating for chromatic dispersion in optical communication networks. Tunable dispersion compensators (TDCs) are used to provide an adjustable (i.e., tunable) amount of dispersion compensation. Previously proposed TDCs include, for example, ring resonators, virtually imaged phased arrays (VIPAs), cascaded Mach-Zehnder interferometers (MZIs), temperature-tuned etalons, waveguide grating routers (WGRs) with thermal lenses, and bulk gratings with deformable mirrors.
The cascaded MZI approach has been found to be particularly promising since it exhibits low loss and can be made with standard silica waveguides in a compact planar lightwave circuit (PLC). However, prior art MZI-based TDCs typically require multiple stages and multiple control voltages, are difficult to fabricate, and have high power consumption, making them complex and expensive.
One prior art design for a TDC is shown in FIG. 1a. The TDC consists of two M-arm interferometers (i.e., waveguide grating routers, WGRs #1 and #2), each consisting of M waveguides (i.e., arms) with adjacent waveguide path-length difference ΔL, and two star couplers. The two WGRs are coupled together at one of their star coupler boundaries with an adjustable lens device. The combination of the two star couplers and the lens can be viewed as an adjustable coupler. The adjustable lens device is a dynamic 2-D element that can provide a quadratic phase distribution −k×2/(2f), where k is the free-space propagation constant, x is the distance along the lens axis, and f is the focal length. f may be controllable to allow tuning of the TDC. The strength, s, of the lens device is defined as s=1/f. When the lens focal length f is equal to the star coupler radius, the coupling is zero, (i.e., each waveguide connected to one of the star couplers couples to only one waveguide of the other star coupler (in a diagonal fashion).
The operation of the TDC in FIG. 1a can be explained as follows: for a given input signal (input from the left), the signal is spread out in wavelength at the lens device by WGR #1. Each spectral portion of the signal impinges on a different portion of the lens device. When the focal length of the lens device is equal to the length of the star coupler radius, all of the spectral portions of the signal are directed such that the field distribution of the spectral portions is centered in the waveguide array in WGR #2. Thus all the spectral portions have the same effective path length in the TDC, and thus, the dispersion of the TDC is zero.
If the focal length of the lens is adjusted to be longer than the radius of the star coupler, longer-wavelengths of the signal (as compared to wavelengths closer to the signal's center wavelength) are predominantly directed toward the shorter waveguides of WGR #2, and shorter-wavelengths are predominantly directed toward the longer waveguides of WGR #2. This results in longer-wavelengths traveling a shorter distance through the TDC than shorter-wavelengths, resulting in the TDC providing negative chromatic dispersion. If the lens focal length is adjusted to be shorter than the star coupler radius, the converse is true and the TDC provides positive chromatic dispersion.
One problem with the prior art TDC of FIG. 1a is that shorter- and longer-wavelengths experience increased loss (i.e., the TDC exhibits a rounded passband) with increased dispersion magnitude because the field distribution of the spectral portions of shorter- and longer-wavelengths are not centered in the waveguide array in WGR #2. Thus, these spectral portions do not couple efficiently into the output waveguide causing the rounded passband.
Another prior art TDC design shown in FIG. 1b solves the passband rounding problem of the TDC of FIG. 1a. The TDC of FIG. 1b has three MZIs each consisting of 2 waveguides, and two adjustable lenses coupling the MZI's. The two ‘outer’ MZIs have an adjacent waveguide path-length difference of ΔL, and the center MZI has a waveguide path-length difference of 2ΔL. The passband of this TDC is not rounded (to the first order) as the dispersion magnitude is increased. However, with such MZI based TDCs the maximum achievable dispersion is substantially limited.